EC-304 Electronic Devices - Nov 2022 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain the breakdown mechanisms in PN diode.
   How to answer

   • Define depletion region and reverse bias breakdown
   • Explain Zener breakdown - tunneling in heavily doped narrow junction
   • Explain avalanche breakdown - impact ionization in lightly doped wide junction
   • Compare voltage levels, temperature coefficients and I-V characteristics
2. 7 Marks
   bThe reverse saturation current Io in a germanium diode is $6 \mu A$. Calculate the current flowing through the diode when the applied forward bias voltages are 0.2, 0.3 and 0.4 V at room temperature.
   How to answer

   • State diode current equation $I = I_o(e^{V/\eta V_T}-1)$ and $V_T \approx 26 mV$ at room temperature
   • Substitute $I_o = 6 \mu A$ and $V = 0.2 V$ to compute current
   • Repeat calculation for $V = 0.3 V$ and $V = 0.4 V$
   • Comment on exponential increase of forward current
3. 7 Marks
   aExplain the following:

   1. i)Varactor diode
   2. ii)Photo diode
   How to answer

   • Define varactor diode as voltage-variable capacitance under reverse bias with structure and C-V relation
   • Give symbol, characteristics and applications like tuning
   • Define photodiode as light-sensitive reverse-biased junction with working principle
   • Give symbol, characteristics and applications like optical detection
4. 7 Marks
   bCompare halfwave rectifier, fullwave rectifier and bridge rectifier.
   How to answer

   • Show circuit configurations of halfwave, centre-tapped fullwave and bridge rectifier
   • Compare on transformer utilization, PIV, ripple frequency, efficiency
   • Compare on number of diodes, cost and DC output
   • Conclude relative merits and applications
5. 7 Marks
   aDefine Clipper. Explain different types of clipper circuits.
   How to answer

   • Define clipper as wave-shaping circuit that clips portion of input
   • Explain series and shunt positive and negative clippers with circuits and waveforms
   • Explain biased clippers and combination clipper
   • Give transfer characteristics and applications
6. 7 Marks
   bDesign LC filter for a Full-wave rectifier circuit to provide an output voltage of 10 V with a load current of 200 mA and the ripple is limited to 2%.
   How to answer

   • State ripple factor formula for LC filter and full-wave ripple frequency
   • Use given $V_{dc}=10 V$, $I_{dc}=200 mA$, ripple $2\%$ to find required L and C
   • Select standard values satisfying critical inductance condition
   • Show filter circuit with rectifier and load
7. 7 Marks
   aCalculate the quiescent current and voltage of collector to base bias arrangement using the following data: $V_{cc} = 10\ V, R_b = 100\ K, R_c = 2\ K, \beta = 50$ and also specify a value of $R_b$ so that $V_{ce} = 7\ V$.
   How to answer

   • Draw collector-to-base bias circuit and write KVL equations
   • Calculate base current, collector quiescent current $I_{CQ}$ and $V_{CEQ}$ for given $R_b$, $R_c$, $\beta$
   • Rearrange bias equation to find $R_b$ for $V_{ce}=7 V$
   • Comment on stabilization
8. 7 Marks
   bFor the NPN transistor connected in CE configuration with $V_{CC} = 9\ V, V_{BB} = 4 V, I_C = 5\ mA, V_{CE} = 5\ V, \beta = 50$ and $V_{BE} = 0.7\ V$. Find $\beta$ and $R_B$.
   How to answer

   • Draw CE configuration with $V_{CC}$, $V_{BB}$, $R_B$, $R_C$ and write input-output KVL
   • Use $I_C = \beta I_B$ and $V_{BB}=I_B R_B+V_{BE}$ to find $R_B$
   • Verify $V_{CE}$ from output loop
   • State value of $\beta$ and $R_B$
9. 7 Marks
   aExplain voltage divider biasing circuit. Discuss the advantages of the circuit.
   How to answer

   • Draw voltage divider bias circuit with R1, R2, RE, RC
   • Explain Thevenin equivalent and stabilization by emitter resistor
   • Derive stability factor and Q-point independence from beta
   • List advantages like thermal stability and single supply
10. 7 Marks
    bDetermine the h-parameters for CE configuration from the characteristics curve.
    How to answer

    • Define h-parameters $h_{ie}, h_{re}, h_{fe}, h_{oe}$ for CE
    • Show input and output characteristics curves
    • Explain graphical determination using slopes at Q-point
    • Write formulas for each parameter
11. 7 Marks
    aIn the CE amplifier calculate the mid frequency voltage gain and lower 3-dB point. The transistor has h-parameters $h_{fe} = 400$ and $h_{ie} = 10\ k\Omega$, the circuit details are $R_s = 600 \Omega, R_L = 5\ k\Omega, R_e = 1\ k\Omega, V_{cc} = 12\ V, R_1 = 15\ k\Omega, R_2 = 2.2\ k\Omega$ and $C_e = 50\ \mu F$.
    How to answer

    • Draw CE amplifier with voltage divider bias and emitter bypass capacitor
    • Calculate mid-frequency voltage gain $A_v = -h_{fe} R_{L}'/h_{ie}$
    • Calculate lower 3-dB frequency due to $C_e$ and coupling capacitors
    • Substitute $R_s, R_L, R_e, R_1, R_2, h_{fe}, h_{ie}$ values
12. 7 Marks
    bExplain the Darlington amplifier circuit. List the applications of the Darlington circuit.
    How to answer

    • Draw Darlington pair using two transistors with collectors tied
    • Explain working as super-beta composite transistor
    • Derive overall current gain, input impedance and voltage gain
    • List applications like power amplifiers, regulators, motor control
13. 7 Marks
    aDiscuss the operation of enhancement MOSFET at different regions with suitable figures and equations.
    How to answer

    • Describe enhancement MOSFET structure with no built-in channel
    • Explain cutoff, linear/triode and saturation regions with $V_{GS}>V_T$
    • Give drain current equations $I_D$ in each region
    • Draw output and transfer characteristics
14. 7 Marks
    bSketch the circuit of CS amplifier and derive the expression for voltage gain at low frequencies.
    How to answer

    • Sketch common-source amplifier with biasing and small-signal model
    • Derive voltage gain using low-frequency equivalent circuit
    • Include effect of coupling and bypass capacitors at low frequencies
    • Give final expression for $|A_v|$ at low frequencies
15. 14 Marks
    Write short notes on any two:

    1. i)PN diode in biased condition
    2. ii)Ebers moll model
    3. iii)Current mirroring circuits
    4. iv)JFET characteristics
    How to answer

    • For each chosen topic give definition, construction/working and diagrams
    • Give relevant equations, characteristics and bias conditions
    • Mention merits, limitations and applications
    • Conclude with comparison where applicable